Elevated intracranial pressure (ICP) is a leading cause of irreversible brain damage after trauma, brain disorder, or disease. In severe cases, disability and death may occur. ICP monitoring provides early warning of the onset of deteriorating conditions which, once recognized and diagnosed, can often be effectively treated to prevent neurological impairment. Certain medical conditions, such as meningitis, encephalitis, Reye's syndrome, diabetic encephalopathy, hepatic encephalopathy, near drowning, hydrocephalus, cerebral infarction, subarachnoid hemorrhage, among others, also require ICP to be monitored. Additionally, the time development of a patient's ICP is often of significance to emergency room physicians.
Currently, ICP monitoring is achieved using either sensors implanted within the cranium or external sensors connected to the measurement site in the cranium with a fluid-filled catheter. Both approaches are invasive, generating risk of intracranial infection and pain for the patient, and require neurosurgical expertise for their implantation. Moreover, long periods of patient movement restriction are often required. Most typical among the invasive procedures is the lumbar puncture where a catheter having a pressure-sensing device is placed in the lumbar subarachnoid space. Variations of this technique include drilling a hole in the skull and inserting the catheter.
Existing techniques for noninvasive estimation of ICP include the assessment of level of consciousness on neurologic examination, opthalmoscopic examination of the optical fundi for evidence of papilledema in adults, and the palpation of the fontanelles and skull sutures in infants. Unfortunately, these clinical methods are highly qualitative, do not necessarily correlate directly with more sophisticated analytic ICP measurements, and cannot be performed on a continuous basis.
Several researchers have investigated the use of ultrasound for noninvasively monitoring ICP with varying success. For example, H. Kuchiwaki et al., in "Continuous Recording of Changes in Intracranial Pressure Using Interference Echoes of Ultrasonic Wave: A Preliminary Report of Practicality and Clinical Evaluation," J. Clin. Ultrasound 20, 447 (1992), have demonstrated that a correlation exists between the amplitude of ultrasound interference echoes and the level of ICP using a combination of pulse-echo, high-speed digitizer and time-windowing techniques. The authors attach the transducers to the surface of the skull bone using cyanoacrylate. No direct means for calibration are provided.
Another ultrasonic technique has been described by John H. Cantrell et al., in "Measuring Intracranial Pressure and Volume Noninvasively," NASA Tech Briefs, p. 78 (June 1994) at the NASA Langley Research Center. An ultrasonic tone-burst is directed through the cranium and the phase of the ultrasound signal is monitored subsequent to its traversal. This phase relates to the time required for the tone burst to be reflected from the inside surface of the cranium. Variation in ICP alters the stress level experienced by the skull bone and affects its characteristic sound propagation. Feed-back loop electronics are employed for determining the phase of the received echo using the same transducer that is used for generating the initial signal from outside the skull. A calibration may be achieved by applying a known amount of external mechanical pressure to the skull. No quantitative data are reported, however. Since only a small area of the skull bone (the area of transducer contact) is probed by the ultrasound, the sensitivity is poor, and sophisticated electronics are required for monitoring small changes in sound velocity in the body of the skull due to changes in pressure. Pulsed or tone-burst techniques also require high voltages (-100 V or higher) for proper excitation.
Accordingly, it is an object of the present invention to provide a noninvasive method for measuring changes in intracranial pressure.
Another object of the invention is to provide a noninvasive method for monitoring intracranial pressure in real time.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.